The Seven Sisters are seven mighty chalk cliffs on the coastline between Eastbourne and Seaford that form a very popular walking route.
I walked it a while ago. I did cheat a bit as I walked from Eastbourne to Belle Tout, stayed the night in the Belle Tout Lighthouse and then walked from there to Seaford. Walking it in one go is doable and depending on your pace it should take between 4 and 6 hours. There are also a few shortcuts you can take.